Walkie-talkie type services have long proved popular amongst users who wish to communicate brief messages quickly between one another. Conventionally, such services have been provided by two-way portable radios which utilise a dedicated part of the radio spectrum, but which only allow users to communicate with a small group of pre-selected users who utilise similar terminals and who are within range of the relatively short operating range of the radios. More recently, services have been introduced into the United States which piggy-back on the existing cellular telephone infrastructure. However, these services have been proprietary in nature and have not allowed users to communicate between different operator networks.
In an attempt to broaden the use of walkie-talkie type services, an industry grouping known as the Open Mobile Alliance (www.openmobilealliance.org) has been established with the aim of standardising suitable protocols which will allow inter-network operability for Warlike-Talkie services offered over cellular networks. The service established by the various standards is known as Push to talk Over cellular (PoC). PoC proposes that associated speech data will be transported over a packet switched access network. In the case of GSM and UMTS, this will be the general packet radio service (GPRS) access network. In other network architectures, analogous packet switched access networks will be utilised for transporting talk data. Push to Talk services may also be offered over circuit switched access networks, although this is not the preferred option.
As already mentioned above, walkie-talkie type services are ideal for communicating brief messages between two or a relatively small number of users. A talk session is likely to consist of a small number of talk bursts sent in both directions. Given the brief nature of the talk sessions, it is desirable to minimise service interruptions which might arise due to operator network performance issues.
Considering now the provision of GPRS packet switched access in GSM networks, this makes use the GSM cell-based radio access network architecture which is illustrated in FIG. 1. Within each cell 1, a Base Transceiver Station (BTS) 2 provides the interface between a mobile station and the radio access network 3. In order to maintain high quality communication links with mobile stations 4 as mobile stations approach and cross cell boundaries, a cell re-selection mechanism is provided for transferring mobile stations from one BTS to another. Cell re-selection may also occur for other reasons, for example to avoid network congestion. BTSs are grouped into sets, with each set being controlled by a Base Station Controller (BSC) 5. The combination of a BSC and the associated BTSs controlled by that BSC is referred to as a Base Station System (BSS). The radio access network 3 is coupled to a GPRS core network 6 which includes Serving GPRS Support Nodes (SGSNs) 7 and a Gateway SPRS Support Nodes (GGSNs) 8 which facilitate packet switched access to other networks, for example the Internet. As with the cells, the BSCs are arranged in a hierarchical architecture, with groups of BSCs being served by a common SGSN.
GSM specifies extremely sophisticated mechanisms for ensuring that circuit switched voice calls (as opposed to packet switched data sessions) are transferred smoothly during a cell re-selection. Indeed, users are normally unaware that a cell re-selection has occurred. The mechanisms for performing cell re-selection over GPRS sessions are not so sophisticated. As currently specified, cell re-selection of a GPRS session may result in a significant interruption in the transfer of talk bursts associated with a PoC service. This applies in respect of cell re-selections for any of the parties involved in a PoC session, not just the currently talking party. With current implementations of GPRS, cell re-selection is a decision left to the mobile stations, and a cell re-selection (assuming that the old and the new BTS are served by the same BSC) can result in an interruption in the delivery of data of around 2 to 3 seconds. Whilst the most recent GPRS specification sets out an improved, network controlled cell re-selection procedure, the interruption is still on the order of 700 ms. The probability of such a cell re-selection occurring during a PoC session is quite high. In the case of a cell re-selection between BTSs served by different BSCs, the interruption is likely to be even greater, on the order of 4 to 5 seconds, although such cell re-selections will happen only infrequently. In certain rare cases, a BTS cell re-selection will not only result in a change in serving BSC, but will also result in a change in the Serving GPRS Support Node (SGSN) within the GPRS network. This can lead to a service interruption on the order of 10 to 15 seconds.
Such delays are likely to be annoying to users and may have a serious detrimental effect on the perceived quality of the service and therefore on the levels of user satisfaction. While the problem may be mitigated by setting the pre-buffering (downloading before starting playback) in the receiving mobile station long enough to be able to cope with cell re-selections without interruption, this is not desirable as latency is the top-prioritised characteristic for the PoC service, and the longer the pre-buffering, the higher the latency.